Method for screening for compounds using MHC class II invariant chain polypeptide and macrophage migration inhibitory factor

ABSTRACT

Methods and compositions for using the MHC class II invariant chain polypeptide, Ii (also known as CD74), as a receptor for macrophage migration inhibitory factor (MIF), are disclosed. These include methods and compositions for using this receptor, as well as agonists and antagonists of MIF which bind to this receptor, or which otherwise modulate the interaction of MIF with CD74 or the consequences of such interaction, in treatment of conditions characterized by locally or systemically altered MIF levels, particularly inflammatory conditions and cancer.

This application is a continuation of U.S. application Ser. No.10/108,383, filed Mar. 29, 2002, which claims priority to U.S.Provisional Application Ser. No. 60/279,435, filed Mar. 29, 2001. Theentirety of those applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and compositions for using theMHC class II invariant chain polypeptide, Ii (also known as CD74), as areceptor for macrophage migration inhibitory factor (MIF), includingmethods and compositions for using this receptor, as well as agonistsand antagonists of MIF which bind to this receptor or which otherwisemodulate the interaction of MIF with CD74 or the consequences of suchinteraction, in methods for treatment of conditions characterized bylocally or systemically altered MIF levels, particularly inflammatoryconditions and cancer.

2. Background of the Technology

Macrophage migration inhibitory factor (MIF), the first cytokineactivity to be described, has emerged to be seen as a critical regulatorof the innate and adaptive immune response¹⁻³. MIF is encoded by aunique gene, and crystallization studies have shown MIF to define a newprotein fold and structural superfamily⁴. Despite the fact that thebiological activity attributed to MIF first was described almost 30years ago, information regarding MIF's precise role in cell physiologyand immunity has emerged only recently^(1-9,18). MIF is centrallyinvolved in macrophage and T cell activation and in the development ofseptic shock, arthritis, and other inflammatory conditions². Also, MIFhas been linked to cancer³².

MIF is critically involved in the expression of innate and acquiredimmunity. MIF is released by a variety of cell types and is a necessaryfactor for the activation or proliferative responses of macrophages¹⁸, Tcells⁶, and fibroblasts⁷. MIF's mitogenic effects proceed via anautocrine/paracrine activation pathway involving the p44/p42 (ERK-1/2)mitogen-activated protein kinase cascade⁷. MIF −/− mice are highlyresistant to endotoxic shock³, and immunoneutralization of MIF confersprotection against septic shock²⁵ and a variety of immuno-inflammatorypathologies such as delayed-type hypersensitivity²⁶, arthritis²⁷, andglomerulonephritis²⁸. MIF's actions on cells also show a number ofunique features. These include a global, counter-regulatory action onglucocorticoid-induced immunosuppression^(5,6), the induction of asustained pattern of ERK-1/2 activation⁷, and functional antagonism ofp53-dependent apoptosis⁶.

MIF's pro-inflammatory properties have been linked to its capacity tocounter-regulate the immunosuppressive effects of glucocorticoids^(5,6),and its interactions with cells have been presumed to require areceptor-based mechanism of action^(7,8) or to reflect a specialized,intracellular mode of action⁹. Numerous in vitro and in vivo studieshave been consistent with MIF acting by engagement of a cell surfacereceptor, however lack of progress toward the identification ofcandidate receptors has prompted interest in either specialized,intracellular modes of action⁹ or the potential biological role of MIF'stautomerase activity^(2,21). There also is evidence that MIF mayfunction as an isomerase⁴.

The MHC class II-associated invariant chain, Ii (CD74)¹⁰, has beenestablished to play an important role in the processing and transport ofMHC class II proteins from the endoplasmic reticulum to the Golgi¹⁰.Most Ii dissociates from the class II complex as antigenic peptides loadonto their class II binding sites. Approximately 2-5% of total cellularIi also is expressed on the cell surface, where it has been shown tofunction as an accessory molecule for T cell activation¹¹. Ii has beenpreviously implicated in signaling and accessory functions for immunecell activation¹¹⁻¹³.

U.S. Pat. No. 5,559,028 to Humphreys, et al. discloses gene constructsfor expression of wild type and mutant Ii chains in recombinant cells.U.S. Pat. No. 5,726,020 to Humphreys, et al. discloses and claimsexpressible reverse gene constructs and oligonucleotides that hybridizewith an Ii mRNA molecule, thereby inhibiting translation of the Ii mRNAmolecule.

SUMMARY OF THE INVENTION

The invention is based in part upon the identification, utilizingexpression cloning and functional analyses, that the Class II-associatedinvariant chain polypeptide, Ii (or CD74)¹⁰, is a cellular receptor forMIF. Thus, MIF binds to the extracellular domain of Ii, a Type IImembrane protein, and Ii is required for MIF-induced cell activationand/or phenotypic changes including, for instance, signaling via theextracellular signal-related kinase (ERK)-1/2MAP kinase cascade and cellproliferation. The inventive relationship provides a mechanism for MIF'sactivity as a cytokine and identify it as a natural ligand for Ii, whichhas been previously implicated in signaling and accessory functions forimmune cell activation.

Accordingly, one aspect of the present invention relates to methods forscreening compounds to identify positive or negative modulators of MIFbinding to, or activity in connection with binding to, CD74. In a firstinstance, such a method comprises a biochemical (i.e., acellular)binding assay, comprising: contacting an MHC class II invariant chain(Ii) polypeptide with MIF in the presence and absence of a testcompound, and comparing the binding interaction of the MIF and Iipolypeptides in the presence of the test compound with their interactionin the absence of the test compound, whereby a compound that positivelymodulates the interaction of MIF with the Ii polypeptide is identifiedas an enhancer of MIF binding activity and a compound that negativelymodulates the interaction of MIF with the Ii polypeptide is identifiedas an inhibitor of MIF binding activity. Enhancers so identified arecandidate therapeutic agonists or enhancers of MIF, whereas inhibitorsso identified are candidate therapeutic antagonists of MIF. Forinstance, a test compound may reinforce the binding of MIF to the Iipolypeptide (i.e., increase the affinity of the interaction) and therebyenhance the interaction of MIF and the Ii polypeptide. Such an enhanceris thereby identified as an agonist or enhancer of MIF, and isidentified as a candidate therapeutic agent to enhance, independently orin connection with endogenous or exogenous MIF, MIF effects in subjectsrequiring such augmentation. Alternatively, a test compound thatcompetes with MIF for binding to the Ii polypeptide or otherwiseinhibits the interaction of the MIF with the Ii polypeptide isidentified as an antagonist of MIF, and is identified as a candidatetherapeutic agent to antagonize MIF effects in subjects requiring suchantagonism. In this biochemical binding assay, the Ia polypeptidecomprises the complete Ia sequence or an MIF-binding fragment thereof,and the assay is conveniently conducted with recombinantly prepared MIFand Ii peptides, one of which is optionally immobilized to a solidsupport, and one of which (or a binding partner thereto, such as anantibody) is labeled to facilitate detection and measurement of theMIF:Ii binding interaction.

In a second aspect, the binding assay may be a cellular binding assay,comprising CD74 expressed (either normally or as a consequence ofgenetic engineering for Ii expression) by a cell (prokaryotic oreukaryotic), typically on the cell surface, and MIF binding thereto isdetected and measured in the presence or absence of a test compound. Asin the above described biochemical or acellular assay, a comparison ismade of the binding interaction of the MIF and the cell-displayed Iipolypeptide in the presence of the test compound with their interactionin the absence of the test compound, whereby a compound that positivelymodulates the interaction of MIF with the Ii polypeptide (i.e.,increases their affinity) is identified as an enhancer of MIF bindingactivity and a compound that negatively modulates the interaction of MIFwith the Ii polypeptide (i.e., decreases their affinity) is identifiedas an inhibitor of MIF binding activity. Enhancers so identified arecandidate therapeutic agonists or enhancers of MIF, whereas inhibitorsso identified are candidate therapeutic antagonists of MIF.

In a third aspect, the cellular assay is a signaling assay, in which theactivity of an intracellular signaling cascade is measured before andafter MIF is contacted to cell-displayed CD74 polypeptide, either in thepresence or the absence of a test compound. Preferably, the signalingassay is an ERK-1/2 activation assay. A test compound that positivelymodulates the signaling activity of MIF via interaction with the Iipolypeptide is identified as an enhancer of MIF signaling activity and acompound that negatively modulates the signaling of MIF via interactionof MIF with the Ii polypeptide is identified as an inhibitor of MIFsignaling activity. Enhancers so identified are candidate therapeuticagonists or enhancers of MIF, whereas inhibitors so identified arecandidate therapeutic antagonists of MIF.

In a fourth aspect, the cellular assay is a cellular activity or cellphenotype assay, in which the activity or phenotype of a target cell ismeasured before and after MIF is contacted to cell-displayed CD74polypeptide, either in the presence or the absence of a test compound.Preferably, the activity or phenotype assay is a proliferation assay oran assay for functional antagonism of p53-dependent apoptosis. A testcompound that positively modulates the chosen cellular activity orphenotypic change mediated by MIF via interaction with the Iipolypeptide is identified as an enhancer of MIF cellular activity and acompound that negatively modulates the chosen cellular activity orphenotypic change mediated by MIF via interaction with the Iipolypeptide is identified as an inhibitor of MIF cellular activity.Enhancers so identified are candidate therapeutic agonists or enhancersof MIF, whereas inhibitors so identified are candidate therapeuticantagonists of MIF.

The invention also provides an enhancer of MIF, including an agonist, oran inhibitor, including an antagonist of MIF, identified by any of themethods above. One form of such an agonist or antagonist would be anantibody or antigen-binding fragment thereof, such as an anti-CD74antibody. Anti-CD74 antibodies and CD74-binding fragments thereof areknown in the art. For instance, the anti-CD74 antibody may be amonoclonal antibody and also may be a human, humanized or chimericantibody, made by any conventional method.

Another aspect of the invention relates to a method of inhibiting aneffect of MIF on a cell comprising on its surface an MHC class IIinvariant chain (Ii) polypeptide which binds MIF and thereby mediatesthe effect of MIF. This method comprises: contacting the cell with anantagonist or other inhibitor of MIF, where the antagonist or inhibitorinhibits, in a first instance, binding of MIF to the Ii polypeptide; ina second instance, signaling initiated by MIF:Ii interaction; and in athird instance, a change in cellular activity, metabolism or phenotypeeffected by MIF:Ii interaction. In any of these methods the antagonistor inhibitor may be an antibody or fragment thereof which binds to theIi polypeptide. Alternatively, the inhibitor may be soluble Iipolypeptide or a soluble MIF-binding fragment thereof which inhibits theinteraction of MIF and Ia polypeptide (or the cellular consequences ofsuch interaction) by binding to MIF or by interacting with Iipolypeptide on the surface of a cell. In some cases, the cell comprisingIi polypeptide is present in a mammal and the antagonist or otherinhibitor is administered to the mammal in a pharmaceutical composition.A mammal that would benefit from this method is a mammal suffering froma condition or disorder characterized by MIF levels locally orsystemically elevated above the normal range found in mammals notsuffering from such a condition. In such a case, the antagonist orinhibitor is administered in an amount effective to treat the conditionor disorder. For instance, the mammal may be suffering from cancer or aninflammatory disorder, and the antagonist or inhibitor is administeredin an amount effective to treat the cancer or inflammatory disorder. Theinflammatory disorder may be, for instance, septic shock or arthritis.

More particularly, one aspect of the invention is a method of inhibitingan activity of MIF, which method comprises: contacting MIF with an MHCclass II invariant chain (Ii) polypeptide or a fragment thereof whichbinds to MIF. The fragment of the MHC class II invariant chain (Ii)polypeptide which binds to MIF may be a soluble form of the polypeptide,particularly a soluble form that comprises the extracellular bindingdomain of this type II transmembrane polypeptide. In some cases, the MIFto be inhibited is in a mammal and the Ii polypeptide or a fragmentthereof is administered to the mammal in a pharmaceutical composition.Where the mammal suffers from cancer or an inflammatory disorder, suchas septic shock or arthritis, the Ii polypeptide or fragment thereof isadministered in an amount effective to treat the disorder. In a furtherinstance, the MIF antagonist or inhibitor is administered in an amounteffective to treat an infectious disease, in which disease MIF or apolypeptide evolutionarily related to MIF (as evidenced by sequencehomology) deriving from the infecting pathogen (whether a virus,bacterial, fungus, or especially, a parasite) is present locally,systemically, or at the host:pathogen interface.

Yet another aspect of the invention relates to a method of purifying MIFcomprising: contacting a sample comprising MIF with an MHC class IIinvariant chain (Ii) polypeptide or a fragment thereof which binds toMIF, under conditions that promote the specific binding of MIF to the Iipolypeptide or fragment thereof, and separating the MIF:Ii polypeptidecomplex thereby formed from materials which do not bind to the Iipolypeptide or fragment thereof. In this method, the Ii polypeptide maybe immobilized on a solid support matrix. The invention also provides amethod of assaying for the presence of MIF comprising: contacting asample with an MHC class II invariant chain (Ii) polypeptide or afragment thereof which binds to MIF under conditions that promote thespecific binding of MIF to the Ii polypeptide or fragment thereof, anddetecting any MIF:Ii polypeptide or MIF:Ii polypeptide fragment complexthereby formed.

Still another method provided by the invention is a method for reducingan effect of MIF on a cell comprising on its surface an MHC class IIinvariant chain (Ii) polypeptide or fragment thereof which binds MIF andthereby mediates the effect of MIF. This method comprises: providing tothe cell an antisense nucleic acid molecule in an amount effective toreduce the amount of Ii polypeptide produced by the cell. The antisensenucleic acid molecule specifically binds to a portion of mRNA expressedfrom a gene encoding the MHC class II invariant chain (Ii) polypeptideand thereby decreases translation of the mRNA in the cell and,ultimately, the level of Ii polypeptide on the surface of the cell. Inthis method the cell comprising the Ii polypeptide may be in a mammal,for instance, a mammal suffering from a condition or disordercharacterized by MIF levels locally or systemically elevated above thenormal range in mammals not suffering from such a condition or disorder.For instance, the mammal may be suffering from a cancer or aninflammatory disorder, such as septic shock or arthritis. In such acase, the antisense nucleic acid is administered in a pharmaceuticalcomposition, in an amount effective to treat the condition or disorder.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates high affinity binding of MIF to THP-1 monocytes. a,Alexa-MIF shows full retention of dose-dependent MIF biological activityas assessed by activation of the p44/p42 (ERK-1/2) MAP kinase cascade,visualized by western blotting of cell lysates using antibodies specificfor phospho-p44/p42 or total p44/p42; and b, suppression ofp53-dependent apoptosis induced by serum starvatuin (CM: completemedium, SFM: serum-free medium). MIF or Alexa-MIF were added at 50ng/ml. Data shown are Mean±SD of triplicate wells and are representativeof 3 independent experiments. Further evidence for the retention ofnative structure by Alexa-conjugation was provided by the measurement ofMIF tautomerase activity using L-dopachrome methyl ester as substrate²⁵.No difference in the tautomerase activity of Alexa-MIF versusunconjugated MIF was observed (Alexa-MIF: ΔOD₄₇₅=0.275 sec⁻¹ μg⁻¹protein, versus rMIF: ΔOD₄₇₅=0.290 sec⁻¹ μg⁻¹; P=NS) c, Flow cytometricanalysis shows the binding of Alexa-MIF to THP-1 monocytes is markedlyenhanced by IFN-δ treatment. Competition for Alexa-MIF binding wasperformed in the presence of 1 μg/ml unlabeled, rMIF d, Directvisualization of Alexa-MIF binding to THP-1 monocytes by confocalmicroscopy THP-1 cells were grown on cover slips, incubated with INFγ (1ng/ml) for 72 hrs and stained with Alexa-MIF (left panel) or Alexa-MIFplus excess, unlabeled rMIF (right panel). Cell bound Alexa-MIF wasrapidly internalized upon shifting cells from 4° to 37° for 15 mins(right panel). Magnification: 630× e, Binding characteristics ofAlexa-MIF to IFNγ-activated, THP-1 monocytes. The inset shows thebinding data transformed by Scatchard analysis, indicating two distinctbinding activities; one with K_(d)=3.7×10⁻⁸ m the other withK_(d)=3.5×10⁵, data are representative of 3 independent experiments.

FIG. 2 show that Ii is a cell surface binding protein for MIF a,Sequential cycles of fluorescence-activated cell-sorting of COS-7 celltransfectants shows enrichment for MIF binding activity b, Diagramsindicating structure of Ii (35 kDa isoform), and three of tenrepresentative cDNA clones with MIF binding activity. IC, TM, and EC arethe intracellular, transmembrane, and extracellular domains. M1 and M7refer to two sites of alternative translation initiation. c, Flowcytometry analysis of MIF binding to Ii-expressing cells. Enhancedbinding of Alexa-MIF to Ii-transfected versus control vector-transfectedCOS-7 cells (left panel), inhibited binding of Alexa-MIF toIi-transfected COS-7 cells incubated with anti-Ii mAb (clone LN2) versusan isotypic mAb control (con mAb) (middle panel), and enhanced bindingof Alexa-MIF to IFNγ-stimulated, THP-1 monocytes incubated with anti-IimAb (clone LN2) versus an isotypic mAb control (right panel). The datashown are representative of at least three independent experiments. Theanti-Ii mAb, LN2 (PharMingen), is reactive with an epitope residingwithin 60 amino acids of the extracytoplasmic, C-terminus of the proteind, MIF binds to the extracellular domain of Ii in vitro. [³⁵S]-Iiprotein was prepared in a coupled transcription and translation reactionutilizing plasmids encoding Ii fragments of different lengths.Protein-protein interaction was assessed by measuring boundradioactivity in 96-well plates that were pre-coated with MIF (n=6 wellsper experiment). The data shown are representative of three experiments,showing that MIF binding is severely compromised with vectors expressingIi fragments of amino acids 1-72 or 1-109 versus robust binding withvectors expressing Ii fragments of amino acids 1-149 or 1-232(full-length).

FIG. 3 illustrates Ii mediation of MIF stimulation of ERK-1/2 (p44/p42)phosphorylation in COS-7 cells a, ERK-1/2 phosphorylation is induced byMIF in COS-7 cells transfected with Ii vector (COS-7/Ii) or controlvector (COS-7/V). Cells were treated without or with various doses ofrMIF for 2.5 hrs and analyzed for phospho-p44/p42 and total p44/p42 bywestern blotting b, There is dose-dependent inhibition of MIF-inducedERK-1/2 phosphorylation by anti-Ii mAb. COS-7 cells were transfectedwith an Ii vector and stimulated with 50 ng/ml MIF for 2.5 hrs in thepresence of an isotypic control mAb or an anti-Ii mAb (clone LN2) atdifferent doses. In control experiments, anti-Ii showed no effect onERK-1/2 phosphorylation in the absence of added MIF (data not shown).

FIG. 4 illustrates western blots of MIF-induced phosphorylation a, MIFdose dependently stimulates ERK-1/2 (p44/p42) phosphorylation in humanRaji B cells, as visualized by western blotting for phospho-p44/p42 b,There is inhibition of MIF-induced ERK-1/2 phosphorylation in Raji cellsby anti-Ii mAb also. Raji cells were stimulated with 50 ng/ml of MIF for2.5 hrs in the presence of an isotype control antibody (Con Ab) or thetwo anti-Ii mAbs,—B741 or LN2, each added at 50 μg/ml. c, Anti-Iiinhibits MIF-induced Raji cell proliferation quantified by ³H-thymidineincorporation d, Anti-Ii inhibits MIF-induced proliferation of humanfibroblasts also. Antibodies were added to a final concentration of 50μg/ml. The results shown are the Mean±SD of triplicate assays and arerepresentative of at least three separate experiments. Anti-Iiantibodies showed no effect on cell proliferation in the absence ofadded MIF (data not shown).

FIG. 5 shows the complete nucleotide sequence (SEQ ID NO: 1) and longesttranslated amino acid sequence (beginning at nt 8; SEQ ID NO:2) of thehuman mRNA for the Ii polypeptide (HLA-DR antigens associated invariantchain p33 [GenBank Accession Nr. X00497 M14765]), as reported inStrubin, M. et al., The complete sequence of the mRNA for theHLA-DR-associated invariant chain reveals a polypeptide with an unusualtransmembrane polarity. EMBO J., 3, 869-872 (1984).

DETAILED DESCRIPTION

The following abbreviations are used herein: Alexa-MIF: Alexa 488-MIFconjugate, ERK: extracellular-signal-regulated kinase, Ii: MHC classII-associated invariant chain (CD74), INFγ: interferon-γ, mAb:monoclonal antibody, MIF: macrophage migration inhibitory factor.

Utilizing expression cloning and functional analyses, we have identifiedas a cellular receptor for MIF the Class II-associated invariant chain,Ii (CD74)¹⁰. MIF binds to the extracellular domain of Ii, a Type IImembrane protein, and Ii is required for MIF-induced cellular effects,including for instance, activation of the ERK-1/2 MAP kinase cascade andcell proliferation. These data provide a mechanism for MIF's activity ascytokine and identify it as a natural ligand for Ii, which has beenpreviously implicated in signaling and accessory functions for immunecell activation¹¹⁻¹³. We linked the fluorescent dye Alexa 488¹⁴ torecombinant MIF by standard techniques, verified the retention ofbiological activity of the conjugate (FIGS. 1A,B), and conducted bindingexperiments with a panel of cell types known to respond to MIF. By wayof illustration, using flow cytometry, we observed high-affinity bindingof Alexa-MIF to the surface of the human monocytic cell line, THP-1.This binding activity was induced by activation of monocytes withinterferon-γ (IFNγ), and was competed by the addition of excess,unlabeled MIF (FIG. 1C). Confocal microscopy and direct visualization ofIFNγ-treated monocytes at 4° C. showed surface binding of Alexa-MIF, andcell-bound Alexa-MIF was internalized upon shifting temperature to 37°C. (FIG. 1D). Quantitative binding studies performed with increasingconcentrations of Alexa-MIF revealed two apparent classes of cellsurface receptors (FIG. 1E). The higher affinity binding activity showeda K_(d) of 3.7×10⁻⁸ M and 3.1×10⁴ binding sites per cell, and the loweraffinity binding showed a K_(d) of 3.5×10⁻⁷ M and 4.9×10⁴ sites percell.

To identify the MIF receptor, we prepared cDNA from IFNγ-activated THP-1monocytes and constructed a mammalian expression library in thelambdaZAP-CMV vector¹⁵. Library aliquots representing a total of 1.5×10⁷recombinants were transfected into COS-7 cells, which we had establishedpreviously to exhibit little detectable binding activity for MIF, andthe transfectants were analyzed by flow cytometry for Alexa-MIF binding.Positively-staining cells were isolated by cell sorting, and the cDNAclones collected, amplified, and re-transfected into COS-7 cells foradditional rounds of cell sorting (FIG. 2A). After four rounds ofselection, single colonies were prepared in E. coli and 250 colonieswere randomly picked for analysis. We sequenced 50 clones bearing cDNAinserts of ≧1.6 kB and observed that 10 encoded the Class II-associatedinvariant chain, Ii (CD74), a 31-41 kD Type II transmembrane protein¹⁶.While the isolated clones differed with respect to their total length,each was in the sense orientation and encoded a complete extracellularand transmembrane domain (FIG. 2B).

To confirm that Ii is a cell surface binding protein for MIF, weanalyzed the binding of Alexa-MIF to COS-7 cells transfected with an Iiexpression plasmid (FIG. 2C). Binding was inhibited by excess, unlabeledMIF (data not shown), and by an anti-Ii mAb directed against theextracellular portion of the protein. Anti-Ii mAb also inhibited thebinding of Alexa-MIF to IFN-γ stimulated THP-1 monocytes. The inhibitionby anti-Ii mAb of Alexa-MIF binding to THP-1 monocytes was significant,but partial, consistent with the interpretation that Ii represents oneof the two classes of cell surface receptors for MIF revealed byScatchard analysis (FIG. 1E). [³⁵S]-Ii protein prepared by a coupledtranscription and translation reticulocyte lysate system bound to MIF invitro, and the principal binding epitope was localized to a 40 aminoacid region contained within the Ii extracellular domain (FIG. 2D).

To verify the functional significance of MIF binding to Ii in anexemplary system, we examined the activity of MIF to stimulate ERK-1/2activation and cellular proliferation in different Ii-expressing cells.We observed an MIF-mediated increase, and a dose-dependent, anti-IimAb-mediated decrease, in ERK-1/2 phosphorylation in Ii-transfectedCOS-7 cells (FIG. 3). Irrespective of Ii gene transfection however, wecould not detect any proliferative effect of MIF on this monkeyepithelial cell line (data not shown). We then examined the activity ofMIF to induce ERK-1/2 activation and downstream proliferative responsesin the human Raji B cell line, which expresses a high level of Ii¹⁹. MIFstimulated the phosphorylation of ERK-1/2 in quiescent Raji cells, andeach of two anti-Ii mAbs blocked this stimulatory effect of MIF (FIGS.4A,B). Of note, the inhibitory effect of anti-Ii on ERK-1/2phosphorylation was associated with a significant decrease in theMIF-stimulated proliferation of these cells (FIG. 4C). Additionally, weconfirmed the role of the MIF-Ii stimulation pathway in cells outsidethe immune system. MIF extends the lifespan of primary murinefibroblasts⁸, and both MIF's mitogenic effects and its induction of theERK-1/2 signal transduction cascade have been best characterized in thiscell type⁷. Fibroblasts express low levels of Ii²⁰, and we observed thatanti-Ii significantly inhibited both ERK-1/2 phosphorylation and themitogenic effect of MIF on cultured fibroblasts (FIG. 4D and data notshown).

In prior experiments, we have experienced considerable difficulty inpreparing a bioactive, ¹²⁵I-radiolabelled MIF, and have observed theprotein to be unstable to the pH conditions employed for biotinconjugation. By contrast, modification of MIF by Alexa 488 at a lowmolar density produced a fully bioactive protein which enabledidentification of MIF receptors on human monocytes, and the expressioncloning of Ii as a cell surface MIF receptor. These data significantlyexpand our understanding of Ii outside of its role in the transport ofclass II proteins, and support recent studies which have described anaccessory signaling function for Ii in B and T cell physiology¹⁰⁻¹³.

These findings provide a first insight into the long sought-after MIFreceptor, although additional proteins are likely involved in someMIF-mediated activities. For instance, like MIF, Ii is a homotrimer²³,and the Ii intracellular domain consists of 30-46 amino acids, dependingon which of two in-phase initiation codons are utilized¹⁶.Monocyte-encoded Ii has been shown to enhance T cell proliferativeresponses, and this accessory function of Ii has been linked to aspecific, chondroitin-sulphate-dependent interaction between Ii andCD44¹¹. We have observed an inhibitory effect of anti-CD44 on ERK-1/2phosphorylation, but not MIF binding, in Ii-expressing cells. This isconsistent with the inference that MIF-bound Ii is a stimulating ligandfor CD44-mediated MAP kinase activation. CD44 is a highly polymorphicType I transmembrane glycoprotein²⁴, and CD44 likely mediates some ofthe downstream consequences of MIF binding to Ii.

Interference in the signal transduction pathways induced by MIF-Iiinteraction, for instance by providing antagonists or inhibitors ofMIF-Ii interaction, offers new approaches to the modulation of cellularimmune and activation responses to MIF. Agents active in this regard(agonists and antagonists and other inhibitors) have predictedtherapeutic utility in diseases and conditions typified by local orsystemic changes in MIF levels.

The specific binding interaction between MIF and the class II invariantchain polypeptide, Ii, also makes convenient the use of labeled MIFreagents as “Trojan horse-type” vehicles by which to concentrate adesired label or toxin in cells displaying cell surface Ii. Briefly, adesired label or toxic entity is associated with an MIF ligand (forinstance, by covalent attachment), and the modified MIF ligand then ispresented to cells displaying cell surface-localized Ii, which class IIinvariant chain polypeptide binds to and causes the internalization ofthe modified MIF ligand, thus causing the operative cell to becomespecifically labeled or toxicated. The Ii-displaying cells may beexposed to the modified MIF ligand in vitro or in vivo, in which lattercase Ii-displaying cells may be specifically identified or toxicated ina patient. A wide variety of diagnostic and therapeutic reagents can beadvantageously conjugated to an MIF ligand (which may be biologicallyactive, full length MIF or an Ii-binding fragment thereof, or a muteinof either of the preceding and particularly such a mutein adapted to bebiologically inactive and/or to be more conveniently coupled to alabeling or toxicating entity), providing a modified MIF ligand of theinvention. Typically desirable reagents coupled to an MIF ligandinclude: chemotherapeutic drugs such as doxorubicin, methotrexate,taxol, and the like; chelators, such as DTPA, to which detectable labelssuch as fluorescent molecules or cytotoxic agents such as heavy metalsor radionuclides can be complexed; and toxins such as Pseudomonasexotoxin, and the like.

Methods

MIF and Antibodies

Human recombinant MIF was purified from an E. coli expression system asdescribed previously²² and conjugated to Alexa 488¹⁴ by themanufacturer's protocol (Molecular Probes, Eugene Oreg.). The averageratio of dye ligand to MIF homotrimer was 1:3, as determined bymatrix-assisted laser-desorption ionization mass spectrometry (Kompactprobe/SEQ, Kratos Analytical Ltd, Manchester, UK). Anti-human Iimonoclonal antibodies (clones LN2 and M-B741) were obtained fromPharMingen (San Jose Calif.).

Flow Cytometry, Scatchard Analysis, and Confocal Microscopy.

THP-1 cells (2.5×10⁵ cells/ml) were cultured in DMEM/10% FBS with orwithout IFNγ (1 ng/ml, R&D Systems, Minneapolis, Minn.) for 72 hrs.After washing, 5×10⁵ cells were resuspended in 0.1 ml of medium andincubated with 200 ng of Alexa-MIF at 4° C. for 45 mins. The cells thenwere washed with ice-cold PBS (pH 7.4) and subjected to flow cytometryanalysis (FACSCalibur, Becton Dickinson, San Jose, Calif.). In selectedexperiments, THP-1 monocytes or COS-7 transfectants were incubated withAlexa-MIF together with 50 μg/ml of an anti-Ii mAb or an isotypiccontrol mAb. For Scatchard analysis, triplicate samples of IFNγ-treated,THP-1 cells (1×10⁶) were incubated for 45 mins at 4° C. in PBS/1% FBStogether with Alexa-MIF (0-1.5 μM, calculated as MIF trimer), washed 3×with cold PBS/1FBS, and analyzed by flow cytometry using CellQuestSoftware (Becton Dickinson, San Jose, Calif.)²⁹. The specific bindingcurve was calculated by subtracting non-specific binding (measured inthe presence of excess unlabeled MIF) from total binding. Confocalfluorescence microscopy of Alexa-MIF binding to cells was performed withan LSM 510 laser scanning instrument (Carl Zeiss, Jena Germany). THP-1cells were incubated with INFγ for 72 hrs and washed 3× with PBS/1% FBSprior to staining for 30 mins (4° C.) with 2 g/μl of Alexa-MIF, orAlexa-MIF plus 50 ng/μl unlabeled, rMIF.

cDNA Library Construction, Expression, and Cell Sorting

cDNA was prepared from the poly(A)⁺ RNA of IFNγ-activated, THP-1monocytes, cloned into the lambdaZAP-CMV vector (Stratagene, La Jolla,Calif.), and DNA aliquots (2.5 μg/ml) transfected into 15×10⁶ COS-7cells by the DEAE-dextran method³⁰. The transfected cells were incubatedwith Alexa-MIF for 45 min at 4° C., washed, and the positively-stainingcells isolated³¹ with a Moflo cell sorter (Cytomation, Fort Collins,Colo.). In a typical run, 1.5×10⁷ cells/ml were injected and analyzed ata flow rate of 1×10⁴ cells/sec. Recovery was generally 90%. Plasmid DNAwas extracted from sorted cells using the Easy DNA kit (Invitrogen,Carlsbad, Calif.) and transformed into E. coli XL-10 gold (Stratagene,La Jolla, Calif.) for further amplification. Purified plasmid DNA thenwas re-transfected into COS-7 cells for further rounds of sorting. After4 rounds of cell sorting, 250 single colonies were picked at random andthe insert size analyzed by PCR. Clones with inserts >1.6 Kb wereindividually transfected into COS-7 cells and the MIF binding activityre-analyzed by flow cytometry.

In Vitro Transcription and Translation

Using a full-length Ii cDNA clone as template, three truncated (1-72aa,1-109aa, 1-149aa) and one full-length (1-232aa) Ii product weregenerated by PCR and subcloned into the pcDNA 3.1/V5-HisTOPO expressionvector (Invitrogen). The complete nucleotide sequence of an exemplary IicDNA clone and the putative Ii polypeptide forms that it encodes arepresented in FIG. 5. The fidelity of vector construction was confirmedby automated DNA sequencing and the constructs then used as template forcoupled transcription and translation using the TNT Reticulocyte Lysatesystem (Promega, Madison Wis.). The binding of [³⁵S]-labeled Ii toimmobilized MIF was assessed by a 3 hr incubation at room temperature,as recommended by the TNT protocol.

Activity Assays.

The dose-dependent phosphorylation of ERK-1/2 was measured by westernblotting of cell lysates using specific antibodies directed againstphospho-p44/p42 or total p44/p42 following methods describedpreviously⁷. MIF-mediated suppression of apoptosis was assessed inserum-deprived, murine embryonic fibroblasts by immunoassay ofcytoplasmic histone-associated DNA fragments (Roche Biochemicals,Indianapolis, Ind.)⁸. Proliferation studies were performed by amodification of previously published procedures^(7,8). Human Raji Bcells (American Type Tissue Culture, Rockville, Md.) were cultured inRPMI/10% FBS, plated into 96 well plates (500-1000 cells/well), andrendered quiescent by overnight incubation in RPMI/0.5% serum. The cellswere washed, the RPMI/0.5% serum replaced, and the MIF and antibodiesadded as indicated. After an additional overnight incubation, 1 μCi of[³H]-thymidine was added and the cells harvested 12 hrs later.Fibroblast mitogenesis was examined in normal human lung fibroblasts(CCL210, American Type Tissue Culture) cultured in DMEM/10% FBS,resuspended in DMEM/2% serum, and seeded into 96 well plates (150cells/well) together with rMIF and antibodies as shown. Isotype controlor anti-Ii mAbs were added at a final concentration of 50 μg/ml.Proliferation was assessed on Day 5 after overnight incorporation of[³H]-thymidine into DNA.

As will be apparent to a skilled worker in the field of the invention,numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that the invention may be practiced otherwise than asspecifically described herein.

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All publications and patent applications mentioned in the specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application had been specifically andindividually indicated to be incorporated by reference. The discussionof the background to the invention herein is included to explain thecontext of the invention. Such explanation is not an admission that anyof the material referred to was published, known, or part of the priorart or common general knowledge anywhere in the world as of the prioritydate of any of the aspects listed above.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and that this application is intended to cover anyvariations, uses, or adaptations of the invention following, in general,the principles of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth.

1. A method for screening compounds for an agonist or antagonist ofmacrophage migration inhibitory factor (MIF) protein comprising:contacting, in vitro, an MHC class II invariant chain (Ii) polypeptidewith said MIF protein in the presence and absence of a candidatecompound, and comparing the binding interaction of the said MIF proteinand said Ii polypeptide in the presence of said candidate compound withtheir binding interaction in the absence of said candidate compound,whereby a candidate compound that enhances the binding interaction ofsaid MIF protein with said Ii polypeptide is identified as an agonist ofMIF protein, and a candidate compound that inhibits the bindinginteraction of said MIF protein with said Ii polypeptide is identifiedas an antagonist of MIF protein; wherein said Ii polypeptide comprisesthe amino acid sequence of SEQ ID NO: 2 or a MIF protein-bindingfragment thereof.
 2. The method of claim 1, wherein said Ii polypeptidecomprises the amino acid sequence of SEQ ID NO:
 2. 3. The method ofclaim 1, wherein said Ii polypeptide comprises the MIF protein-bindingfragment of the amino acid sequence of SEQ ID NO:
 2. 4. The method ofclaim 3, wherein said MIF protein-binding fragment comprises amino acids73-232 of SEQ ID NO: 2.